In the next section of this presentation, we'll look at designing two-wire or loop powered transmitters. We'll go over the theory of operation, derive the transfer function for each circuit, and look at design examples and go over key things to consider when you're designing a two-wire transmitter.
This is the circuit for the two-wire transmitter or loop powered transmitter. We'll now go over the theory of operation and derive the transfer function for this circuit. You can see that it consists of a DAC, op amp, and voltage regulator. This voltage regulator is to regulate the high input voltage from the loop plus down to the operating voltage of the DAC and op amp.
So the first step in analyzing this circuit is applying the ideal op amp theory that the voltage at the inverting node and non-inverting node of A1 is equivalent. We can then see that by doing a KCL into the non-inverting node, that the current flowing across R2 is VREG over R2, and the current flowing in from the DAC is VDAC over R1. We know that there's no current flow into the op amp. So all of this current is flowing through R3. So i3 is equal to i1 plus i2.
So we have a local ground that is actually floating. It can be kind of confusing, but it's important to realize the distinction between the ground of the loop voltage supply and this floating ground. So the return current or the quiescent current from the regulator, op am, and DAC actually flow back up through this ground and down through R4 because all of the current entering Loop plus must flow out of Loop minus.
The additional current is regulated by Q1. So we have this current, iBJT, flowing through Q1 and R5. And we have the quiescent current flowing back up through the floating local ground through R4 out Loop minus. So we know that i4 is the sum of this quiescent current flowing back up through ground plus the current through the BJT.
We know that the voltage across R3 and R4 is the same. So we can calculate i4 as i1 plus i2 times R3 over R4. And we know that the current flowing out of the Loop minus will be the sum of these two. So here we have our overall transfer function for this circuit.
There are a couple of considerations when designing a two-wire transmitter. One of the first things to realize is that R2 is responsible for setting the offset current or the 4 milliamps. The current contribution does not change based on the DAC code. So it's set solely by the regulator voltage and the value of R2.
The output of the DAC does vary this input current to the non-inverting node by VDAC over R1. So this is responsible for the span or setting that 16 milliamp range from 4 to 20 milliamps. I1 and i2 sum together, and then they're gained up by R3 and R4, based on 1 plus R3 over R4. So we typically see very high gain stages. And we want a high gain stage, a high ratio of R3 to R4 so that most of the current is flowing through the BJT and the current is minimized through R1 and R2.
Just as in the three-wire transmitter case, we need to keep compliance voltage in mind for the two-wire transmitter. So in normal operation, Q1 is operating in the forward active region. And we need to make sure that we have enough compliance voltage for it to stay in this operating region to regulate the loop current properly.
But normally, in the two-wire transmitter case, the limitation comes from the input voltage of the regulator, the minimum input voltage. So normally, the regulator is going to drop out before there's a headroom issue for Q1. So in this regulator, the minimum input voltage is violated. It will essentially stop powering the DAC and op amp.
But there are some ways for the BJT to approach the cutoff region. And that's based on R5. So R5 is a resistor for stability. And so you need to make sure that R5 isn't too large to eat into the headroom for the transistor.
We have some design examples in the form of TI reference designs. The first one is this fully discrete design that's focused on low cost. So it uses DAC7311, OPA317, and the TL431 shunt regulators to regulate the loop voltage.
It's focused, as I said, on cost. So these are fairly low-cost devices. It's essentially implementing the circuit that we saw on the previous slide with the discrete approach to regulating the loop current. You can find more information about this in the reference design guide for TIPD158.
We also have a TI design that is partially discrete. And it functions as the previous design, but instead of discreetly implementing the current regulation, we use XTR116, which is a voltage to current converter designed specifically for loop powered transmitters. So it's designed to be paired with a DAC in a sensor transmitter application. There are a couple of alternate XTRs, XTR115 and 117, with slightly different parameters. You can view more information about this design in the reference design guide of TIPD190.
The only fully integrated single chip solution that we offer is DAC161S997. This uses a delta sigma DAC and an integrated current output stage to regulate the loop current. And some of the advantages are it has low air drift, low power consumption. And one of the main overall advantages is that it's, essentially, a single chip solution for the loop powered transmitter.